Method for in situ Preparation of Antimony-doped Silicon and Silicon Germanium films

ABSTRACT

A process for forming an antimony-doped silicon-containing layer includes: (a) depositing by chemical vapor deposition the antimony-doped silicon-containing layer above a semiconductor structure, using an antimony source gas and a silicon source gas or a combination of the silicon source gas and a germanium source gas; and (b) annealing the antimony-doped silicon-containing layer at a temperature of no greater than 800° C. The antimony source gas may include one or more of: trimethylantimony (TMSb) and triethylantimony (TESb). The silicon source gas comprises one or more of: silane, disilane, trichlorosilane, (TCS), dichlorosilane (DCS), monochlorosilane (MCS), methylsilane, and silicon tetrachloride. The germanium source gas comprises germane.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional patent application of U.S.patent application (“Parent Application”), Ser. No. 16/506,682, entitled“Method for in situ Preparation of Antimony-doped Silicon and SiliconGermanium films,” filed on Jul. 9, 2019, which relates to and claimspriority of U.S. provisional patent application (“ProvisionalApplication”), Ser. No. 62/695,334, entitled “Method for in situPreparation of Antimony-doped Silicon and Silicon Germanium films,”filed on Jul. 9, 2018. The disclosure of the Provisional Application ishereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to semiconductor circuit fabrication. Inparticular, the present invention relates to in situ formation ofantimony-doped silicon or silicon germanium films.

2. Discussion of the Related Art

Three-dimensional (3-D) memory cells, such as those used in commerciallyavailable NAND memory, make use of doped polycrystalline silicon(polysilicon) films in the transistor, diode, resistor and otherdevices. It is advantageous to be able to dope the polysilicon films insitu (i.e., concurrently with the deposition of the polysilicon film),as compared to doping them ex situ (i.e., performed after deposition ofthe polysilicon film). Examples of ex situ doping techniques include ionimplantation. In situ doping requires fewer processing steps than exsitu doping, and enables greater control of the distribution of thedopant atoms within the silicon or polysilicon film. Examples of dopantsthat have commonly been deposited by in situ doping of silicon areboron, phosphorus, and arsenic. It is important to note that in situdoped silicon films can be deposited as polycrystalline films, or asamorphous films, and then crystallized at a later step by annealing thedeposited, amorphous film at an elevated temperature above 550° C.Finished 3-D memory devices most commonly include many layers of dopedpolycrystalline films. In this detailed description, although thesilicon films are referred to as polysilicon (i.e., polycrystallinesilicon) films, such silicon films may initially be deposited asamorphous or polycrystalline films.

Antimony is an example of an n-type dopant in silicon that hasheretofore not been in situ deposited in silicon. Antimony has twoadvantageous properties that make its use in silicon-based 3-D memorycircuits attractive: its low diffusivity and its low temperature ofactivation. FIG. 1 is a concentration-depth profile analysis of arsenicin a silicon film performed using secondary ion mass spectrometry(SIMS). To prepare the profile of FIG. 1, a 30 nm thick silicon film wasimplanted with arsenic ions. After a cleaning step that removed anyoxide (e.g., SiO₂) layer on the 30-nm silicon film, a 120 nm thicksilicon film was deposited on top of the arsenic-implanted silicon film.Portions of the combined films were then annealed at either 750° C. for4 hours or 1000° C. for 5 seconds. FIG. 1 shows SIMS analyses on arsenicdiffusion in the combined film (i) before annealing (i.e., asimplanted), (ii) after annealing at 750° C. for 4 hours and (iii) afterannealing at 1000° C. for 5 seconds. FIG. 2 is a concentration—depthprofile analysis of antimony in a silicon film performed using SIMS. Toprepare the profile of FIG. 2, a 30 nm thick silicon film was implantedwith antimony ions. After a cleaning step to remove any SiO₂ layer onthe 30 nm silicon film, a 120 nm silicon film was deposited on top ofthe antimony-implanted silicon film. Portions of the combined films wereannealed at either 750° C. for 4 hours or 1000° C. for 5 seconds. FIG. 2shows SIMS analyses on antimony diffusion in the combined film (i)before annealing (i.e., as implanted), (ii) after annealing at 750° C.for 4 hours and (iii) after annealing at 1000° C. for 5 seconds. FIGS. 1and 2 show that antimony diffuses more slowly than arsenic.

Most dopant atoms in silicon “activate.” (Activation is the process bywhich a dopant atom replaces a silicon atom in the lattice, therebyproviding a charge carrier). Unlike many other common dopant atoms,antimony does not necessarily activate more readily in polysilicon withincreasing activation temperature. See, e,g., “The annealing behavior ofantimony implanted polycrystalline silicon,” by J. L. Tandon, H. B.Harrison, C. L. Neoh, K. T. Short, and J. S. Williams, Applied PhysicsLetters 40 (1982), 3, pp. 228-230, and “Sub-50-nm Dual-Gate Thin-FilmTransistors for Monolithic 3-D Flash,” by Andrew J. Walker, IEEETransactions on Electron Devices 56, no 11 (2009), pp. 2703-2710.Annealing temperatures of between 600 and 800° C. have been shown to beoptimal for activation and achieving lowest sheet resistance.

Low diffusivity is important for obtaining and maintaining throughoutthe fabrication process sharp delta-like junctions in the devices, andfor enabling small devices. A low activation temperature also providesthe same advantages by minimizing the diffusion of other dopants, suchas boron, in the devices. In situ deposition of antimony-doped films hasbeen achieved in molecular beam epitaxy (MBE). However, MBE is not anappropriate mass production technique and does not have good stepcoverage. (Step coverage is the ability of a deposited film to evenlydeposit on a substrate that contains significant topography or steps.)For example, a deposition technique with good step coverage would allowa film deposited inside a trench to have roughly the same thickness onthe sidewalls of the trench as on the horizontal surfaces above thetrench.

Recently developed gas sources for metal-organic chemical vapordeposition (MOCVD) of antimony-containing III-V and II-VI semiconductorcompounds enable in situ antimony doping of silicon films.

SUMAMRY

The present invention provides a method for depositing in situantimony-doped silicon, as well as some uses in 3-D memory circuits.

According to one embodiment of the present invention, a process forforming an antimony-doped silicon-containing layer includes: (a)depositing by chemical vapor deposition the antimony-dopedsilicon-containing layer above a semiconductor structure, using anantimony source gas and a silicon source gas or a combination of thesilicon source gas and a germanium source gas; and (b) annealing theantimony-doped silicon-containing layer at a temperature of no greaterthan 800° C. The antimony source gas may include one or more of:trimethylantimony (TMSb) and triethylantimony (TESb). The silicon sourcegas comprises one or more of: silane, disilane, trichlorosilane, (TCS),dichlorosilane (DCS), monochlorosilane (MCS), methylsilane, and silicontetrachloride. The germanium source gas comprises germane.

In one embodiment, the antimony-doped silicon-containing layer isdeposited into a cavity formed by removal of a sacrificial layer.

A process of the present invention may further include: (a) forming ann-type semiconductor layer above the semiconductor structure; and (b)annealing the n-type semiconductor layer at a temperature higher than900° C., prior to annealing the antimony-doped silicon-containing layer.

The process of the present invention may be used to form a storagetransistor, which includes (a) first and second n-doped polysiliconlayers serving, respectively as a drain region and a source region ofthe storage transistor; (b) one or more in situ antimony-dopedsilicon-containing layers of the present invention, at least one ofwhich being adjacent to one of the first and second n-doped polysiliconlayers; (c) an intrinsic or lightly-doped p-doped semiconductor layerbetween the first and second n-doped polysilicon layer, serving aschannel region for the storage transistor; (d) a conductor serving as agate electrode for the storage transistor; and (e) a charge storagelayer between the intrinsic or lightly-doped p-doped semiconductorlayer. In one embodiment, the storage transistor shares the first andsecond n-doped polysilicon layers with an adjacent like storagetransistor. In that embodiment, the channel region for the storagetransistor and the channel region for the like storage transistor areseparated by a p-doped semiconductor layer. In the formation of thestorage transistor, the n-doped polysilicon layers are annealed at atemperature higher than 900° C., prior to annealing the antimony-dopedsilicon-containing layer at a lower temperature no greater than 900° C.

The n-doped polysilicon layers may also be antimony-doped.

The present invention is better understood upon consideration of thedetailed description below in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows SIMS analyses on arsenic diffusion in an implanted siliconfilm (i) before annealing (i.e., as implanted), (ii) after annealing at750° C. for 4 hours and (iii) after annealing at 1000° C. for 5 seconds.

FIG. 2 shows SIMS analyses on antimony diffusion in an implantedsilicon, film (i) before annealing (i.e., as implanted), (ii) afterannealing at 750° C. for 4 hours and (iii) after annealing at 1000° C.for 5 seconds.

FIG. 3 shows a cross section of 3-D memory structure 300 that includesin situ antimony-doped silicon films, according to one embodiment of thepresent invention.

FIGS. 4A and 4B illustrate fabricating N⁺ common source regions andcommon drain regions in a 3-D memory structure using a “replacementprocess”, in accordance with one embodiment of the present invention.

To simplify the detailed description, like elements in the figures areassigned like reference numerals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Antimony-doped silicon films can be deposited by low pressure chemicalvapor deposition (LPCVD) techniques. In such an LPCVD process, sourcegases of antimony may include, for example, trimethylantimony (TMSb) andtriethylantimony (TESb). See, e.g., the articles: (i) “The metal-organicchemical vapor deposition and properties of III-V antimony-basedsemiconductor materials,” by Robert Biefield, Materials Science andEngineering R 36 (2002), pp. 105-142; and (ii) “Room temperatureoperation of In_(x)Ga_(1-x)Sb/InAs type II quantum well infraredphotodetectors grown by MOCVD,” by D. H. Wu, Y. Y. Zhang, and M.Razeghi, Applied Physics Letters 112 (2018), pp. 111103-111107. Sourcegases of silicon include silane, disilane, trichlorosilane, (TCS),dichlorosilane (DCS), monochlorosilane (MCS), methylsilane, and silicontetrachloride. Source gases of germanium include germane.

Flowing one or more antimony source gases with one or more siliconsource gases, or with a combination of silicon and germanium sourcegases, over a target substrate at a temperature between 400° C. and 900°C., preferably no greater than 800° C., and a pressure between 1 and4000 mTorr, deposits antimony-doped silicon films or antimony-dopedsilicon germanium films. While common dopant gases for in situ doping ofsilicon are chlorides or hydrides—namely boron tri chloride (BCl₃),diborane (B₂H₆), phosphine (PH₃), and arsine (AsH₃)—precursors to thesedopant gases (e.g., trimethylboron (TMB)) have been successfully used inMOCVD to in situ dope silicon. One example such a MOCVD precursorprocess is disclosed in “Structural and electrical properties oftrimethylboron-doped silicon nanowires,” by K.-K. Lew et al., AppliedPhysics Letters 85, 15 (2004) pp. 3101-3103. Helium, nitrogen, and argonare other gases that may also be included with the silicon- andantimony-containing gases.

FIG. 3 shows a cross section of 3-D memory structure 300 that includesin situ antimony-doped silicon films, according to one embodiment of thepresent invention. FIG. 3 shows four memory cells in memory structure300. Specifically, FIG. 3 shows that adjacent memory cells T1 a and T2 aare formed above adjacent memory cells T1 b and T2 b, with memory cellsT1 a and T2 a being isolated from memory cells T1 b amd T2 b bydielectric layer 15 b. Memory cells T1 a and T2 a share common sourceregion 10 a and common drain region 10 b, which are both n⁺ doped silionlayers (e.g., in situ antimony-doped silicon layers). Likewise, memorycells T1 b and T2 b also share common source and drain regions, whichare also respectively labeled 10 a and 10 b in FIG. 3, indicating thatthey are constituted and formed in like manner as the n⁺ doped silionlayers associated with common source region 10 a and common drain region10 b of memory cells T1 a and T2 a.

Memory cells T1 a, T2 a, T1 b and T1 b include p⁻ channel regions 30-1a, 30-2 a, 30-1 b and 30-2 b, respectively, provided between respectivecommon source regions 10 a and common drain regions 10 b. Channelregions 30-1 a and 30-2 a are separated laterally from each other byp-type material 20. Likewise, channel regions 30-1 b and 30-2 b areseparated from each other laterally by p-type material 20 Over channelregions 30-1 a, 30-2 a, 30-1 b and 30-2 b are provided charge storagematerial 40 and conductors 50 for formation of gate electrodes. In FIG.3, the length A of each channel region are reduced—relative to non insitu doped silicon films—because diffusion of antimony atoms from the insitu antimony-doped silicon films (i.e., common source region 10 a andcommon drain region 10 b) into p⁻-doped channel regions 30-1 a, 30-2 a,30-1 b and 30-2 b is minimal, as already suggested from theconcentration-depth profile of FIG. 2. Were it not the case, n-typedopant diffusion into the p⁻-doped channels may overwhelm the p-typedoping, causing the p⁻-doped channel region to become n-type, therebydestroying the transistor. A reduced channel length (i.e., length A)provides memory structure 300 with a lower aspect ratio¹ and a higherchannel current each memory cell. ¹ In this context, the aspect ratio ofa feature is its height divided by its width

FIGS. 4A and 4B illustrate fabricating n⁺ common source regions andcommon drain regions in a 3-D memory structure using a “replacementprocess”, in accordance with one embodiment of the present invention. Inthe replacement process of FIGS. 4A and 4B, a sacrificial material isfirst provided in memory structure 400 to stand in place of commonsource region 10 a and common drain region 10 b, until the final form ofmemory structure 300 is almost complete. Use of sacrificial materials isdescribed in greater detail in copending U.S. patent application(“Copending Application”), Ser. No. 15/248,420, entitled“Capacitive-Coupled Non-volatile Thin-film Transistor NOR Strings inThree-Dimensional Arrays,” filed on Aug. 26, 2016. The disclosure of theCopending Application is hereby incorporated by reference in itsentirety.

FIG. 4A shows partially fabricated memory structure 400, in which eachof p-type layers 60 is fabricated adjacent each of sacrificial materiallayers 10 t. Each of sacrificial material layers 10 t is to be replacedat a subsequent step by an antimony-doped silicon layer, which becomeseither one of common source regions 10 a or one of common drain regions10 b. (In some embodiments, the layers in FIGS. 4A and 4B labeled byreference numeral 20 need not be a p-type materal, as described abovewith respect to FIG. 3, but may be provided as a dielectric material.)After p-type layers 60 are formed and before the channel regions areformed, p-type layers 60 are annealed at an elevated temperature (e.g.1000° C.), which enables the dopant atoms in p-type layers 60 to becomehighly activated. By annealing at a high temperature before fabricationof other layers (e.g., the channel regions), boron diffusion into suchareas (e.g., the channel regions) is avoided. After anneling,sacrificial material layers 10 t are removed and replaced by in situantimony-doped silicon layers. As stated earlier, antimony atoms can behighly activated at a lower temperature (e.g. 750° C.), thus avoidingsignificant n-type dopant diffusion into p-type regions (e.g., thechannel regions).

FIG. 4B shows memory structure 400, after sacrificial material layers 10t are replaced by in situ antimony-doped silicon layers to providerespective common source regions 10 a and common drain regions 10 b.FIG. 4b also shows channel regions 30, charge storage material 40 andconductors 50 designated for forming gate electrodes. In memorystructures 300 and 400 above, p-type layers 20 in FIG. 3 or p-typelayers 60 in FIGS. 4A and 4B are provided to supply additional charagecarriers (i.e., holes) to the channel regions. The additional chargecarriers may be helpful in erase operations. P-type layers 20 in FIG. 3may have a different dopant concentration from channel regions 30-1 a,30-2 a, 30-1 b and 30-2 b, but are connected electrically to channelregions 30-1 a, 30-2 a, 30-1 b and 30-2 b to supply additional chargecarriers to the channel regions. In situ doping of both p⁺ and n⁺polysilicon or polysilicon-germanium layers are particularly suited forreplacement processes used in forming multi-layer memory structures, asthe high aspect ratios in such memory structures preclude ex situ dopingprocesses, such as ion implantation. In situ antimony-doped silicon orsilicon-germanium layers are particularly suitable for applications inwhich a p⁺ layer is desired in the vicinity of, or in contact with a p⁻doped channel region or with an n⁺ source or drain layer. In suchapplications, boron dopant diffusion cannot be tolerated and highactivation of the p⁺ layer is desirable.

In some embodiments, p⁺-doped silicon germanium “hole donor” layers aremore advantageous than p⁺-doped silicon “hole donor” layers, because oftheir more favorable etching rates. For a given etchant, silicongermanium and silicon films can achieve different etch rates. Ingeneral, a silicon germanium film tends to etch at a faster rate than asilicon film. For example, when p-type layer 20 is implemented by ap⁺-doped silicon germanium layer, the silicon germanium layer can berecessed by a selective etch to allow silicon channel regions 30-1 a,30-2 a, 30-1 b and 30-2 b to be accommodated in the recesses. In theetch step to form the recesses, common source regions 10 a and commondrain regions 10 b are not etched, or minimally etched, in comparison top⁺-doped silicon germanium layer 20.

Generally, in situ doped silicon germanium films may also beadvantageous for decreasing the thermal budget relative to in situ dopedsilicon films. For example, while amorphous silicon films crystallize at550° C., amorphous germanium films crystallize at 400° C. Amorphoussilicon germanium films crystallize at a temperature between 400° C. and550° C., depending on the relative fractions of silicon and germanium.Also, dopants activate in silicon germanium films at typically lowertemperatures than in silicon films.

In situ antimony-doping allows precise control of the metallurgicaldoping concentration², from 1.0×10¹⁶ cm⁻³ to 1.0×10²¹ cm⁻³, dependingon: the relative ratio of the antimony source gas or gases to thesilicon source gas or gases used, and the temperature and the pressureduring deposition. For example, a low concentration in situantimony-doped ² While antimony can be doped metallurgically to aconcentration greater than 1.0×10²⁰ cm⁻³, the active concentration isgenerally limited to no greater than 5.0×10¹⁹ cm⁻³. Hence, the term“metallurgical doping concentration” is used more often than the term“active doping concentration”. silicon film (e.g., 1.0×10¹⁷ cm⁻³) may beused as a channel region film in a transistor, while a higherconcentration in situ antimony-doped silicon film (e.g., 1.0×10¹⁹ cm⁻³)may be used in the source and drain regions of the transistor.

The above detailed description is provided to illustrate specificembodiments of the present invention and is not intended to be limiting.Numerous modifications and variations within the scope of the presentinvention are possible. The present invention is set forth in theaccompanying claims.

We claim:
 1. A storage transistor, comprising: first and second n-dopedpolysilicon layers serving, respectively, as a drain region and a sourceregion of the storage transistor; one or more in situ antimony-dopedsilicon-containing layers, at least one of the antimony-dopedsilicon-containing layer being adjacent one of the first and secondn-doped polysilicon layers; an intrinsic or lightly-doped p-dopedsemiconductor layer between the first and second n-doped polysiliconlayer, serving as channel region for the storage transistor; a conductorserving as a gate electrode for the storage transistor; and a chargestorage layer between the intrinsic or lightly-doped p-dopedsemiconductor layer.
 2. The storage transistor of claim 1, wherein thestorage transistor shares the first and second n-doped polysiliconlayers with an adjacent like storage transistor, wherein the channelregion for the storage transistor and the channel region for the likestorage transistor are separated by a p-doped semiconductor layer. 3.The storage transistor of claim 1, wherein each of the antimony-dopedsilicon-containing layer are formed using a process comprising the stepsof: exposing a surface of a semiconductor structure; and flowing overthe surface of the semiconductor structure an antimony source gas and asilicon source gas or a combination of the silicon source gas and agermanium source gas, so as to deposit by chemical vapor deposition anantimony-doped silicon-containing layer at a temperature no greater than900° C.
 4. The storage transistor of claim 3, wherein the antimonysource gas comprises one or more of: trimethylantimony (TMSb) andtriethylantimony (TESb).
 5. The storage transistor of claim 3, whereinthe silicon source gas comprises one or more of: silane, disilane,trichlorosilane, (TCS), dichlorosilane (DCS), monochlorosilane (MCS),methylsilane, and silicon tetrachloride.
 6. The storage transistor ofclaim 3, wherein the germanium source gas comprises germane.
 7. Thestorage transistor of claim 3, wherein the antimony-dopedsilicon-containing layer is deposited into a cavity formed by removal ofa sacrificial layer.
 8. The storage transistor of claim 3, the processfurther comprising: forming an n-type semiconductor layer above thesemiconductor structure; and annealing the n-type semiconductor layer ata temperature higher than 800° C., prior to annealing the antimony-dopedsilicon-containing layer.
 9. The storage transistor of claim 3, whereinthe temperature is no greater than 800° C.
 10. The storage transistor ofclaim 3, wherein the temperature is no greater than 750° C.
 11. Thestorage transistor of claim 3, wherein antimony-doped silicon-containinglayer has a concentration between 1.0×10¹⁶ cm⁻³ and 1.0×10²¹ cm⁻³. 12.The storage transistor of claim 3, wherein antimony-dopedsilicon-containing layer has a concentration between 1.0×10¹⁶ cm⁻³ and1.0×10¹⁹ cm⁻³.
 13. The storage transistor of claim 1, wherein at leastone of the first and second n-doped polysilicon layers includes antimonyas a dopant.